Mailing machines including postage metering systems often use optical mechanisms to sense the presence of an envelope in the requisite position before commencing printing of a postal indicium including the postage amount. Optical mechanisms may also be used to measure a physical dimension of a mail piece being processed by a mailing machine. One such postage metering machine is the DM 500 mailing machine available from Pitney Bowes Inc. of Stamford, Conn. The DM 500 mailing machine may accept envelopes that are typically already addressed and stuffed by the user. The envelopes are inserted at a feed end and are carried along a deck by a transport mechanism. Optical sensors detect presence of the envelope as it moves along the deck and trigger the transport means to move the envelope in position for the postage to be printed thereon. Furthermore, an optical sensor mechanism may be disposed along the transport mechanism path and used to measure a physical dimension such as the width of a mail piece such as a letter or a flat envelope. Such a physical dimension measurement may be used in determining the required applicable postage value using dimensional rating.
Such mailing machines are subject to widely varying ambient light conditions, such as when positioned near an office window. At some times of the workday the ambient light which leaks internal to the machine derives primarily if not fully from artificial lighting in the office, but mid-day on sunny days the ambient light level can be much higher from additive sunlight. Ambient light ‘leaks’ into the machine through a lateral gap which is the area in which the optical presence sensors are disposed (typically either along the deck or opposite it). The optical sensors are typically continuously scanned in order to detect presence of an envelope along the deck, but this varying ambient light condition in some cases cause the sensor to signal to the mailing machine that an object is present when in fact the sensor has detected only the elevated ambient light from the mid-day sun. In effect, sunlight saturates the sensors making the controlling software think that an envelope is covering them, which results in the system randomly starting up when no mail is present on the deck. Such an erroneous detection may be described as a false-detection.
In such cases, end users may be forced to draw blinds in a room, reorient the machine or place their postage metering machines in interior rooms to eliminate the high ambient light conditions from sunlight which trigger the false-detections noted above. But relying on end users to resolve a machine error may be problematic and limiting the placement of a mailing machine may not be practical for a particular installation. Accordingly, there is a need for a mailing machine having an optical sensor that more reliably functions in varying lighting conditions. The exemplary embodiments of the invention disclosed in the present application address several needs including at least the above mentioned false-detection issue.